docs: Gemini 리뷰 스타일 가이드 추가

Author: pooreumjung

.gemini/styleguide.md

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+# Gemini Code Assist Review Guide
+
+## Language
+- Always write review comments in Korean.
+- Keep comments direct, practical, and specific to the current codebase.
+
+## Review Priority
+- Prioritize findings in this order: bugs, data consistency, security, performance, design, maintainability, readability.
+- Focus on backend behavior, API compatibility, persistence behavior, integration risks, and missing tests.
+- Avoid nitpicks and preference-only comments unless they hide a real maintainability risk.
+
+## Project Context
+- This is a Spring Boot backend project.
+- Use Java 21 and Gradle.
+- Follow the existing package structure, naming, and style.
+- Prefer existing services, repositories, DTOs, response conventions, and package boundaries over new abstractions.
+- Keep suggested changes narrowly scoped to the pull request.
+
+## Backend Review Rules
+- Verify Controller, Service, Repository, DTO, and Entity responsibilities are separated clearly.
+- Business logic should live in the appropriate Service or domain model, not in controllers.
+- Entity state changes should use clear behavior methods instead of uncontrolled public setters.
+- Check soft delete behavior, nullable policies, enum expansion, and DB constraints for consistency.
+- Avoid duplicating business rules, payload construction, validation checks, formatting, or helper logic across services.
+- Suggest a shared service/helper only when it removes meaningful duplication, reduces real complexity, or matches an existing local pattern.
+- Check exception handling for consistency between business exceptions and system exceptions.
+- Avoid meaningless `RuntimeException` usage and unnecessary try-catch blocks.
+- Review transaction boundaries, `readOnly` usage, exception propagation, and rollback behavior carefully.
+- When reviewing notification, messaging, FCM, or external side effects, verify failures do not unintentionally roll back the main business operation unless intended.
+- Check for unnecessary queries, N+1 risks, lazy loading issues, missing indexes, and inefficient count/existence checks.
+- For user-specific state, prefer ID-based repository queries when loading the full entity is unnecessary.
+- Check race condition risks, duplicate creation risks, and whether DB unique constraints support application-level validation.
+- Verify authentication, JWT handling, refresh token storage, external token validation, and sensitive logging.
+- If secrets, tokens, DB passwords, JWT secrets, or other sensitive values appear in code, logs, screenshots, or config, warn clearly.
+- Verify request/response DTO usage and avoid returning entities directly from APIs.
+- For backend/frontend integration changes, separate what the backend expects, what the frontend must send, and what environment/config values must match.
+
+## Test Review
+- Check whether new behavior has focused tests.
+- Prefer focused tests before broad tests for narrow changes.
+- Test method names must be written in English.
+- Test comments should use `given`, `when`, `then` only when comments are necessary.
+- If tests cannot be executed due to local infrastructure or existing unrelated failures, state that separately from issues caused by the PR.
+
+## Review Style
+- Lead with findings ordered by severity.
+- Use severity labels such as `[P1]`, `[P2]`, and `[P3]` when helpful.
+- For each finding, explain the problem, why it matters, and a concrete improvement.
+- Include the affected file, method, or behavior whenever possible.
+- Keep each review comment concise and actionable.
+- Use one comment per issue.
+- Do not repeat the same issue in multiple comments.
+- If suggesting code, keep the snippet minimal.
+- Do not wrap the entire review comment in a code block.
+- If a suggestion is optional, say so explicitly.
+- If there are no issues, say that clearly and mention any remaining test gaps or residual risk.
+
+## Commenting Convention
+- Use `FIX ME` only when the code has a real potential issue but immediate exception handling or implementation is intentionally deferred.
+- Use `FIX ME` for unclear business requirements that must be revisited.